A modem is a device for sending and receiving data, generally digital data, travelling via telephone lines.
For sending or receiving data, it is usual to use telephone lines which are used in addition for other uses such as normal telephone communications. This is the case, in particular, for television decoders, such as those using the digital video transmission by satellite standard "DVBS".
These decoders can be linked automatically, via the modem, to a programme supplier without the user of the telephone line being informed at the moment of the communication.
In general, in order not to disturb the usual use of the telephone line, the modem should be such that it gives priority to the telephone communications, that is to say that, if the line is busy, the modem should not attempt the connection and, if appropriate, it should disconnect itself if the user seeks to use the line.
The modem should thus have available a device for detecting the line acquisition or the busy state of the telephone line.
For this detection, it is known to arrange a detector of the variation in current or voltage on the line linked to the modem. However, the variations in current or voltage caused by a telephone handset acquiring the line are often small and therefore difficult to detect. Moreover, the signal detected has to be applied to the modem via a galvanic separator such as an optical coupler if the telephone line conductors should never be earthed, although such a condition is not necessarily imposed on the modem. Hence, the known detection is complex and expensive.